At today’s physics colloquium, Tony Leggett gave a rather striking talk about tests of quantum mechanics. It was striking because, if I understood him correctly, about five years ago he thought it likely that quantum mechanics (specifically, the superposition principle) would break down at macroscopic levels. It’s a very interesting, uncommon, and refreshing point of view.
Apparently, he’s much less certain of this now, in part because of the experiments done over the past few years with superconducting qubits. He still regards Bell inequality violations in such systems as important, however.